1. Field of the Invention
Embodiments of the invention relate to display device, and more particularly to a liquid crystal display device. Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for improving black brightness.
2. Description of the Related Art
Generally, a liquid crystal display device displays a picture corresponding to a video signal using a matrix of liquid crystal cells, which are defined between crossing gate lines and data lines. Each pixel area includes a thin film transistor (hereinafter, referred to as ‘TFT’) connected to a pixel electrode, a gate line and a data line. The TFT switches a video signal from the data line through the TFT to the pixel electrode of a liquid crystal cell in response to a gate drive signal from the gate line. Further, the liquid crystal display device includes a gate drive circuit and a data drive circuit for supplying the drive signals to the gate lines and the data lines.
FIG. 1 is a schematic diagram of the related art liquid crystal display device. As shown in FIG. 1, the liquid crystal display device of the related art includes a liquid crystal panel 800 having a matrix of liquid crystal cells, a gate driver 600 for driving gate lines GL1 to GLn of the liquid crystal panel 800, a data driver 400 for driving data lines DL1 to DLm of the liquid crystal panel 800, and a timing controller 200 for controlling the gate driver 600 and the data driver 400.
The timing controller 200 generates control signals GDC and DDC, which control the gate driver and the data driver. The gate control signals GDC generated in the timing controller 200 includes a gate start pulse GSP, a gate shift clock signal GSC, and a gate output enable signal GOE. The data control signals DDC generated in the timing controller 200 includes a source start pulse SSP, a source shift clock signal SSC, a source output enable signal SOE, and a polarity control signal POL. Further the timing controller 200 supplies a pixel data signal RGB to the data driver 400.
The gate driver 600 sequentially supplies scan signals to the gate lines GL1 to GLn in response to the gate control signals GDC. Thin film transistors along one horizontal line are driven for a horizontal period in response to the scan signal of the gate driver 600. The gate lines GL1 to GLn can receive the scan signals in sequence.
The data driver 400 converts the inputted pixel data into an analog pixel signal and supplies the analog pixel signals of one horizontal line to the data lines DL1 to DLm for each horizontal period while the scan signal is supplied to the gate line GL along the horizontal line. In the alternative, the data driver 400 can convert the pixel data into the analog pixel signal by use of gamma voltages supplied from a gamma voltage generating part (not shown).
The liquid crystal panel 800 includes an upper substrate (not shown) and a lower substrate (not shown) that face each other with a layer of liquid crystal molecules (not shown) therebetween. The upper substrate includes color filters (not shown), a black matrix (not shown) located between the color filters (not shown), and a common electrode (not shown) that supplies a reference voltage to the layer of liquid crystal molecules (not shown). Further, the lower substrate includes a thin film transistor 120 which is formed in each sub-pixel area defined by the crossing of the gate lines GL1 to GLn and the data lines DL1 to DLm, and a pixel electrode 100 connected to the thin film transistor 120. In response to scan signals from the gate line GL1 to GLn, the thin film transistor 120 supplies the pixel signal from the data line DL1 to DLm to the pixel electrode 100. The pixel signal on the pixel electrode 100 drives the layer of liquid crystal molecules between the common electrode and the pixel electrode 100 to control light transmittance through the layer of liquid crystal molecules toward a color filter.
The liquid crystal display device of the related art has one pixel that is a combination of sub-pixels, which are disposed in parallel to realize red R, green G and blue B colors. Each of the red R, green G and blue B sub-pixels makes only allows about 27%˜33% of the amount of the light from the backlight unit BL exit to the upper substrate. Thus, light from the backlight unit BL may not be fully utilized.
The liquid crystal display device is mainly classified into a TN (twisted nematic) mode in which an electric field oriented in a vertical direction between the substrates is used to drive the liquid crystal molecules, and an IPS (in-plane switch) in which an electric field oriented in a horizontal direction between the substrates is used to drive the liquid crystal molecules. In the TN mode, the liquid crystal molecules are driven by a vertical electric field between the common electrode, which is disposed on the upper substrate, and the pixel electrode, which is disposed on the lower substrate. The TN mode has the advantage of a high aperture ratio but has the disadvantage of a narrow viewing angle. In the IPS mode, the liquid crystal molecules are driven by a horizontal electric field between the pixel electrode and the common electrode, which are both disposed in parallel on the lower substrate. The IPS mode has the advantage of a wide viewing angle but the disadvantage of a low aperture ratio.
FIG. 2 is a diagram representing a cross-section of one sub-pixel in the related art IPS mode liquid crystal panel. More particularly, FIG. 2 is a diagram representing a cross-section of one sub-pixel in the related art IPS mode liquid crystal panel 800 shown in FIG. 1. As shown in FIG. 2, a black matrix 2, a color filter 6, an overcoat layer 7, a spacer 13 and an upper alignment film 8 are sequentially formed on a surface of the upper substrate 1 and a transparent electrode material (not shown) is formed on the other surface of the upper substrate 1 to prevent static electricity. A thin film transistor, a common electrode 4, a pixel electrode 100 and a lower alignment film 10 are formed on a lower substrate 5. Liquid crystal molecules (not shown) are injected into a space between the upper plate and the lower plate.
The black matrix 2 is formed to overlap a TFT area of the lower substrate 5, gate line areas and data line areas (not shown). The black matrix defines a cell area ‘A’ where a color filter 6 is to be formed. The black matrix 2 prevents light leakage and absorbs an external light so as to increase contrast.
The color filter 6 is formed over and the black matrix 2 and within the cell area ‘A’. Further, the color filter 6 is formed for each of red (R), green (G), and blue (B) colors to realize R, G, and B colors. An overcoat layer 7 is formed to cover the color filter 6 for planarization purposes. An upper alignment film 8 is formed on the overcoat layer 7. The upper/lower alignment films 8 and 10 are formed by spreading an alignment material, such as polyimide, and performing a rubbing process. A spacer 13 maintains a cell gap between the upper substrate 1 and the lower substrate 5.
The TFT, including a gate electrode 16, is formed on the lower substrate 5 together with the gate line (not shown), a semiconductor layer 126, which overlaps the gate electrode with a gate insulating film 129, and source/drain electrodes 128 and 130 formed together with the data line (not shown) with the semiconductor layer 126 therebetween. The TFT supplies the pixel signal from the data line to the pixel electrode 100 in response to the scan signal from the gate line. The pixel electrode 100 is a transparent conductive material through which light transmittance is high. The pixel electrode contacts the drain electrode 130 of the TFT.
The common electrode 4 is formed in a stripe shape to alternate with the pixel electrode 100. A common voltage being a reference when driving liquid crystal is supplied to the common electrode 4. The liquid crystal is rotated on the basis of a horizontal direction by a horizontal electric field created by the common voltage and a pixel voltage supplied to the pixel electrode 100.
In the liquid crystal panel 800, the light applied from a backlight (not shown) mounted on a lower part of the liquid crystal panel passes through the color filter 6 through the cell area ‘A’ where the pixel electrode 100 and the common electrode 4 are formed, thereby generating R, G, and B colors. The amount of the light passing through the cell area ‘A’ is controlled by the rotation of the liquid crystal molecules, which results from the electric field supplied to the common and pixel electrodes 4 and 100, as described above.
FIG. 3 is a diagram representing a state of light leakage in a related art IPS mode liquid crystal panel. As described above, the amount of light passing through the cell area ‘A’ is controlled by the rotation of the liquid crystal molecules due to the electric field between the pixel electrode 100 and the common electrode 4. The response speed of the liquid crystal molecules is slower than the switching speed of the electrodes, thus the liquid crystal molecules of the cell area ‘A’ do not completely block the light applied from the backlight, as shown in FIG. 3, thereby causing the problem of black brightness.
In general, the response speed of liquid crystal molecules is 5˜10 ms, and the response speed cannot be as fast as the switching speed of the electrodes. Thus, a problem occurs in that a very high black brightness appears because the light of the backlight is not completely blocked and a part of the light is transmitted during a black state in which all of the light of the cell area ‘A’ should be blocked. Such a phenomenon is more pronounced in the IPS mode liquid crystal display device where the common electrode 4 and the pixel electrode 100 are formed in parallel on the lower substrate.
The reason why such a problem is more pronounced in the related art IPS mode liquid crystal display device is because the black brightness cannot be counteracted by a strong reverse electric field since the black color is the initial alignment state for the related art IPS mode liquid crystal display device. In other words, when going to black in the related art IPS mode liquid crystal display device, the related art liquid crystal panel does not change to completely block the light, but rather lets part of the light from a backlight unit through. Accordingly, there is a problem in that the black brightness of the related art liquid crystal panel in the related art IPS mode liquid crystal display device is increased.